EECS 3214: Computer Network Protocols and Applications

Transcription

1 EECS 3214: Computer Network Protocols and Applications Suprakash Datta Course page: Office: LAS datta [at] cse.yorku.ca These slides are adapted from Jim Kurose s slides. EECS S.Datta 1-1

2 Chapter 2: Application layer 2.1 Principles of network applications 2.2 Web and HTTP 2.3 FTP 2.4 Electronic Mail SMTP, POP3, IMAP 2.5 DNS EECS S.Datta 2

3 Some network apps Web Instant messaging Remote login P2P file sharing Multi-user network games Streaming stored video clips Social networking Internet telephony Real-time video conference Massive parallel computing Search EECS S.Datta 3

4 Creating a network app Write programs that run on different end systems and communicate over a network. e.g., Web: Web server software communicates with browser software No software written for devices in network core Network core devices do not function at app layer This design allows for rapid app development application transport network data link physical application transport network data link physical application transport network data link physical EECS S.Datta 4

5 Application architectures Client-server Peer-to-peer (P2P) Hybrid of client-server and P2P EECS S.Datta 5

6 Client-server architecture server: always-on host permanent IP address server farms for scaling clients: communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other EECS S.Datta 6

7 Pure P2P architecture no always on server arbitrary end systems directly communicate peers are intermittently connected and change IP addresses Highly scalable: self scalability new peers bring new service capacity, as well as new service demands But difficult to manage EECS S.Datta 7

8 Hybrid of client-server and P2P Napster File transfer P2P File search centralized: Peers register content at central server Peers query same central server to locate content Instant messaging Chatting between two users is P2P Presence detection/location centralized: User registers its IP address with central server when it comes online User contacts central server to find IP addresses of buddies EECS S.Datta 8

9 Processes communicating Process: program running within a host. within same host, two processes communicate using inter-process communication (defined by OS). processes in different hosts communicate by exchanging messages Client process: process that initiates communication Server process: process that waits to be contacted Note: applications with P2P architectures have client processes & server processes EECS S.Datta 9

10 Sockets process sends/receives messages to/from its socket socket analogous to door sending process shoves message out door sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process API: (1) choice of transport protocol; (2) ability to fix a few parameters (lots more on this later) application process socket application process controlled by app developer transport network link physical Internet transport network link physical controlled by OS EECS S.Datta 10

11 Addressing processes For a process to receive messages, it must have an identifier A host has a unique32- bit IP address Q: does the IP address of the host on which the process runs suffice for identifying the process? Answer: No, many processes can be running on same host Identifier includes both the IP address and port numbers associated with the process on the host. Example port numbers: HTTP server: 80 Mail server: 25 More on this later EECS S.Datta 11

12 App-layer protocol defines Types of messages exchanged, eg, request & response messages Syntax of message types: what fields in messages & how fields are delineated Semantics of the fields, ie, meaning of information in fields Rules for when and how processes send & respond to messages Public-domain protocols: defined in RFCs allows for interoperability eg, HTTP, SMTP Proprietary protocols: eg, Skype EECS S.Datta 12

13 What transport service does an app need? Data integrity some apps (e.g., file transfer, web transactions) require 100% reliable data transfer other apps (e.g., audio) can tolerate some loss Timing some apps (e.g., Internet telephony, interactive games) require low delay to be effective Throughput some apps (e.g., multimedia) require minimum amount of bandwidth to be effective other apps ( elastic apps ) make use of whatever bandwidth they get security encryption, data integrity, EECS S.Datta 13

14 Transport service requirements of common apps Application Data loss Bandwidth Time Sensitive file transfer Web documents real-time audio/video stored audio/video interactive games instant messaging no loss no loss no loss loss-tolerant loss-tolerant loss-tolerant no loss elastic elastic elastic audio: 5kbps-1Mbps video:10kbps-5mbps same as above few kbps up elastic no no no yes, 100 s msec yes, few secs yes, 100 s msec yes and no EECS S.Datta 14

15 Internet transport protocols services TCP service: connection-oriented: setup required between client and server processes reliable transport between sending and receiving process flow control: sender won t overwhelm receiver congestion control: throttle sender when network overloaded does not provide: timing, minimum bandwidth guarantees UDP service: unreliable data transfer between sending and receiving process does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee Q: why bother? Why is there a UDP? EECS S.Datta 15

16 Internet apps: application, transport protocols Application remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] HTTP (e.g., YouTube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype) Underlying transport protocol TCP TCP TCP TCP TCP or UDP TCP or UDP EECS S.Datta 16

17 Securing TCP TCP & UDP no encryption cleartext passwds sent into socket traverse Internet in cleartext SSL provides encrypted TCP connection data integrity end-point authentication SSL is at app layer Apps use SSL libraries, which talk to TCP SSL socket API cleartext passwds sent into socket traverse Internet encrypted See Chapter 7 EECS S.Datta 17

18 Chapter 2: Application layer Next: Ch. 2.2 Web and HTTP Examine the web infrastructure EECS S.Datta 18

19 Web and HTTP First some jargon Web page consists of objects Object can be HTML file, JPEG image, Java applet, audio file, Web page consists of base HTML-file which includes several referenced objects Each object is addressable by a URL Example URL: host name path name EECS S.Datta 19

20 HTTP overview HTTP: hypertext transfer protocol Web s application layer protocol client/server model client: browser that requests, receives, displays Web objects server: Web server sends objects in response to requests HTTP 1.0: RFC 1945 HTTP 1.1: RFC 2068 PC running Explorer Mac running Navigator Server running Apache Web server EECS S.Datta 20

21 HTTP overview (continued) Uses TCP: client initiates TCP connection (creates socket) to server, port 80 server accepts TCP connection from client HTTP messages (applicationlayer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server) TCP connection closed HTTP is stateless server maintains no information about past client requests aside Protocols that maintain state are complex! past history (state) must be maintained if server/client crashes, their views of state may be inconsistent, must be reconciled EECS S.Datta 21

22 HTTP connections Nonpersistent HTTP at most one object sent over TCP connection connection then closed downloading multiple objects required multiple connections HTTP/1.0 uses nonpersistent HTTP Persistent HTTP Multiple objects can be sent over single TCP connection between client and server. HTTP/1.1 uses persistent connections in default mode EECS S.Datta 22

23 Nonpersistent HTTP Suppose user enters URL 1a. HTTP client initiates TCP connection to HTTP server (process) at on port 80 (contains text, references to 10 jpeg images) time 2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates that client wants object somedepartment/home.index 1b. HTTP server at host waiting for TCP connection at port 80. accepts connection, notifying client 3. HTTP server receives request message, forms response message containing requested object, and sends message into its socket EECS S.Datta 23

24 Nonpersistent HTTP (cont.) 5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects 4. HTTP server closes TCP connection. time 6. Steps 1-5 repeated for each of 10 jpeg objects EECS S.Datta 24

25 Response time modeling Definition of RTT: time to send a small packet to travel from client to server and back. Response time: one RTT to initiate TCP connection one RTT for HTTP request and first few bytes of HTTP response to return file transmission time total = 2RTT+transmit time initiate TCP connection RTT request file RTT file received time time time to transmit file EECS S.Datta 25

26 Persistent HTTP Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate host resources for each TCP connection but browsers often open parallel TCP connections to fetch referenced objects Persistent HTTP server leaves connection open after sending response subsequent HTTP messages between same client/server are sent over connection Persistent without pipelining: client issues new request only when previous response has been received one RTT for each referenced object Persistent with pipelining: default in HTTP/1.1 client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects EECS S.Datta 26

27 HTTP request message two types of HTTP messages: request, response HTTP request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) carriage return, line feed at start of line indicates end of header lines carriage return character line-feed character GET /index.html HTTP/1.1\r\n Host: www-net.cs.umass.edu\r\n User-Agent: Firefox/3.6.10\r\n Accept: text/html,application/xhtml+xml\r\n Accept-Language: en-us,en;q=0.5\r\n Accept-Encoding: gzip,deflate\r\n Accept-Charset: ISO ,utf-8;q=0.7\r\n Keep-Alive: 115\r\n Connection: keep-alive\r\n \r\n EECS S.Datta 27

28 HTTP request message: general format EECS S.Datta 28

29 Uploading form input Post method: Web page often includes form input Input is uploaded to server in entity body URL method: Uses GET method Input is uploaded in URL field of request line: EECS S.Datta 29

30 Method types HTTP/1.0 GET POST HEAD asks server to leave requested object out of response HTTP/1.1 GET, POST, HEAD PUT uploads file in entity body to path specified in URL field DELETE deletes file specified in the URL field EECS S.Datta 30

31 HTTP response message status line (protocol status code status phrase) data, e.g., requested HTML file header lines HTTP/ OK\r\n Date: Sun, 26 Sep :09:20 GMT\r\n Server: Apache/ (CentOS)\r\n Last-Modified: Tue, 30 Oct :00:02 GMT\r\n ETag: "17dc6-a5c-bf716880"\r\n Accept-Ranges: bytes\r\n Content-Length: 2652\r\n Keep-Alive: timeout=10, max=100\r\n Connection: Keep-Alive\r\n Content-Type: text/html; charset=iso \r\n \r\n data data data data data... EECS S.Datta 31

32 HTTP response status codes In first line in server->client response message. A few sample codes: 200 OK request succeeded, requested object later in this message 301 Moved Permanently requested object moved, new location specified later in this message (Location:) 400 Bad Request request message not understood by server 404 Not Found requested document not found on this server 505 HTTP Version Not Supported EECS S.Datta 32

33 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: telnet cis.poly.edu 80 Opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. Anything typed in sent to port 80 at cis.poly.edu 2. Type in a GET HTTP request: GET /~ross/ HTTP/1.1 Host: cis.poly.edu By typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server 3. Look at response message sent by HTTP server! (or use Wireshark to look at captured HTTP request/response) EECS S.Datta 33

34 User-server state: cookies Many major Web sites use cookies Four components: 1) cookie header line in the HTTP response message 2) cookie header line in HTTP request message 3) cookie file kept on user s host and managed by user s browser 4) back-end database at Web site Example: Susan access Internet always from same PC She visits a specific e- commerce site for first time When initial HTTP requests arrives at site, site creates a unique ID and creates an entry in backend database for ID EECS S.Datta 34

35 Cookies: keeping state (cont.) Cookie file ebay: 8734 client usual http request msg usual http response + Set-cookie: 1678 server server creates ID 1678 for user Cookie file amazon: 1678 ebay: 8734 one week later: Cookie file amazon: 1678 ebay: 8734 usual http request msg cookie: 1678 usual http response msg usual http request msg cookie: 1678 usual http response msg cookiespecific action cookiespectific action EECS S.Datta 35

36 Cookies (continued) What cookies can bring: authorization shopping carts recommendations user session state (Web e- mail) how to keep state : protocol endpoints: maintain state at sender/receiver over multiple transactions cookies: http messages carry state aside Cookies and privacy: cookies permit sites to learn a lot about you you may supply name and to sites search engines use redirection & cookies to learn yet more advertising companies obtain info across sites EECS S.Datta 36